Roof Leak Traced and Sealed with a FLIR One Thermal Camera on a Flat-Roof Extension

Water rarely drops straight down after it gets through a flat-roof membrane. It can move across the deck, meet a joint or fastener, settle in a low spot, and only then find the gap that leads indoors. That is why the brown mark above a sofa can send a repair in the wrong direction. On a flat-roof extension, the path is even harder to read from below because insulation and service holes hide the route. The FLIR One is useful in that gap between suspicion and demolition: the clip-on phone module reads roughly minus 20C to 120C, overlays a thermal image on a phone photo, and picks up the temperature contrast caused by damp material holding and losing heat differently from dry material.

Why the stain was in the wrong place

On this extension, the visible stain sat close to an internal wall. The membrane failure was later confirmed beside a rooflight upstand, well over two metres away from the mark on the ceiling.

The deck fall, the shallow slope built into flat roofs to move water toward outlets, had carried ingress sideways under the insulation before it dropped through a service penetration. Opening the plasterboard directly below the stain would have exposed dry timber and left an electrician wondering why the leak seemed to have vanished.

Thermal imaging helped because evaporating moisture cools a surface, while saturated insulation affects the speed at which the ceiling below returns to room temperature. In ideal conditions, the FLIR One can pick up gradients as fine as a tenth of a degree, although outdoor practical resolution is coarser. The timing matters: a mild afternoon with little temperature difference across the roof buildup produces a screen full of vague colour.

Getting an image worth trusting

The scan worked because the roof and the interior were at least ten degrees apart. Early morning after a cold night is one useful window. Late evening can also work after the sun has loaded heat into the membrane. Flat calm weather helps, since wind strips away the surface temperature difference the camera is trying to read.

Start indoors. Stand under the affected ceiling and scan the plasterboard around the stain, across the internal wall line, and toward any obvious service routes. That gives a first map of where the ceiling is behaving differently from the surrounding area.

Then take the camera onto the roof and scan the membrane surface. A wet section of insulation can read cooler than dry neighbours on a warm deck. After a cold period, the relationship may reverse, so note the direction of the temperature difference before treating a colour as evidence.

The FLIR One MSX feature matters here. It blends visible-light edges over the thermal frame, so the outline of a rooflight, outlet, parapet, or render joint sits on top of the heat map. Without those edges, a cool area is difficult to place accurately; with them, the same area can be tied to a particular fitting or lap.

Use Iron or Grey for the palette. Rainbow makes smooth gradients look like hard borders, and that can send the search toward edges that only exist on the screen. Once the temperature range of interest is visible, lock the span manually. Auto-scaling changes the colours as the phone pans, which makes two adjacent frames awkward to compare.

Work across the deck as a grid. Take overlapping frames and keep a fixed reference in view where possible, such as a parapet corner. On a small extension roof of about fifteen square metres, a dozen frames can cover the surface well enough to relate one image to the next.

In this case, the anomaly appeared as a cool crescent on the upstream side of the rooflight kerb. That location matched the later physical finding: a lifted lap in the felt at the upstand.

Before cutting or stripping anything, put a moisture meter on the suspect area or use it from the underside where access allows. The thermal image chooses the test zone. The pin meter adds a number, which helps separate a genuinely wet patch from a shaded or thermally bridged surface.

The rooflight detail that failed

Rooflight kerbs fail more often than the open field of a flat-roof membrane. The felt has to turn up the kerb, sit properly under a capping, and survive weather at an awkward change of plane.

A hurried installation can leave a lap ready to peel during the first hard frost. That was the defect found beside the rooflight on this extension.

What the trace changed financially

A FLIR One Gen 3 costs around 200 to 250 pounds. The newer FLIR One Edge Pro costs more.

Set that against a contractor opening and reinstating plasterboard on a hunch. Two ceilings cut, skimmed, dried, and repainted can pass 800 pounds before the leak has been found. Because the water may have travelled sideways, the first opening can miss the actual route completely.

The repair here was modest once the target was known: strip the failed lap at the rooflight upstand, prime the area, and dress a torch-on patch under the existing capping. Materials came in under 60 pounds. Labour was reduced to a single roofer visit because the location no longer had to be guessed.

The camera paid for itself against one avoided exploratory ceiling opening. It also remained available for the next defect, and older extensions often provide another one sooner or later.

There is a resale angle, although it can be overstated. A dated record showing the leak located, sealed, and checked with before-and-after thermal frames gives a cleaner survey trail than a vague damp note. Surveyors treat active ingress seriously, and repair imagery gives a buyer something concrete instead of an open question that gets priced into an offer.

Where the phone camera runs out of reach

The FLIR One belongs in diagnostics. Survey work on a large roof quickly exposes its limits.

The Gen 3 thermal sensor is 160 by 120 pixels. That is workable on a fifteen-square-metre extension when the camera is about a metre from the surface. It is a poor match for a full commercial flat roof viewed from a cherry picker. The field of view is narrow as well, so a large area means many frames and a lot of mental stitching.

Long-standing moisture changes the job. If the deck timber has rotted, sealing a lap is only part of the situation; structural boarding may need to be cut out and replaced. A thermal frame can guide attention to the wet zone, yet rot and failed adhesion in a fully saturated membrane may extend beyond what looks obvious from above.

Standing water also ruins the reading. A puddle equalises surface temperature and hides whatever sits below it. The roof needs to be dry, or at least drained, before the scan has much value.

Interstitial condensation is the false positive that most easily imitates ingress. A cold plume on the ceiling can come from a thermal bridge at a wall junction, or from trapped moisture linked to a vapour-control layer issue that never involved rainwater. Temperature alone cannot identify the source.

Rainfall timing helps. If the anomaly grows within hours of a downpour, rain ingress becomes a stronger explanation. If the same mark sits unchanged through dry weeks, the trail points more toward a cold spot or condensation problem, and torch-on felt will not address that cause.

The unresolved part in this case sat below the visible surface: the cool crescent marked wet material beside the upstand, but the image alone could not prove whether the first breach was above the membrane or inside the roof buildup.